In the dental, medical and veterinary field, the now commonly used syringes are of two kinds: disposable and non-disposable. Disposable syringes are economically practical, that is they are cheap and of simple design, typically consisting of just two parts made from plastic. However, many drugs to be injected come in pre-sterilized single use cartridges which, in general, cannot be used in disposable syringes. As the syringe has to be more complicated to adapt it for use with the cartridge, it is more expensive to manufacture and hence it is non-disposable. Many re-usable syringes which incorporate facility for the manual or for automatic aspiration are even more complicated and they commonly are made from metal, which is usually chrome plated brass.
Self-aspirating syringe for use with drug cartridge is described in the U.S. Pat. No. 3,583,399. Since its invention, self-aspirating syringe have became popular among dentists and are now used world wide. The syringe described in the patent has a thumb rest on the end of a plunger rod. The now popular design of this syringe has a thumb ring on the end of plunger rod, rather than a thumb rest. The syringes of the prior art are made by machining them from metal tubing or from solid metal bars.
Such prior art self-aspirating syringes, however, still have drawbacks. A dentist, while holding the syringe in fingers of his (or her) hand, has to insert the needle in a precisely selected spot in the patient's gum for injection of local anesthetic. Selection of the spot with precision is important, because the needle must not hit a blood vessel. To use the syringe the dentist places the thumb in the thumb ring which in self-aspirating syringes of the prior art looks like a piece cut off from a metal tube. Having a cylindrical inside surface and thin edges makes it uncomfortable to hold.
It is quite common that the dentist works with an assistant. After completion of injection of local anesthetic the dentist swings his (or her) arm sideways to hand over the syringe to the assistant. The dentist's thumb at this time is still trapped in the thumb ring. While the assistant grabs the syringe by its barrel and pulls it off, the thin edge of the thumb ring may press strongly into the thumb, hurting it or else causing a discomfort.
In use, the needle, mounted on the end of syringe barrel, punctures the diaphragm in the drug cartridge. It has been observed that in the self-aspirating syringe it is much more likely that the drug may leak through the puncture. Leakage may not occur every time, it could be very small and not readily visible, yet, nonetheless, it can have important consequences. Even a tiny leak can be a potential carrier of infection from the blood of one patient into the blood of another. While the standard procedure of sterilization of syringes between patients prevents the potential, it still can come into effect by accident.
Dentists usually have more than one syringe and all of them could be identical. On the average, a dentist administers injections to about one thousand patients a year. A human error is possible and a used syringe may be mistaken for a sterilized one.
Another problem with non-disposable syringes of the prior art is corrosion. Exposure to the drug, which is corrosive to metals, to steam and to sterilization temperatures eventually corrodes syringes made from chrome plated brass and the plating flakes off. While the syringe parts made from stainless steel alleviate this problem, stainless steel is more expensive to use.